In order to understand the effect of impurities on boiling point, one must first understand what boiling point is. The boiling point of a solution is generally defined as the the temperature at which the vapor pressure of a liquid is equal to the pressure of the gas above it. It is important to note that a "normal boiling point" of a solution is the temperature at which vapor pressure from the liquid will equal one atmosphere. It is at this temperature, that the liquid vapor will be released as a gas into the atmosphere. Importantly, the boiling point of a solution remains the same even if more heat is added after it starts to boil.
Adding impurities to a solution, in most cases, increases the boiling point of the solution. This occurs because the presence of impurities decreases the number of water molecules available to become vaporized during boiling. Once this occurs, it takes a greater amount of heat to cause the same amount of impure solution to vaporize as it would take to cause a pure solution to vaporize, thus raising the solution's overall boiling point. It is important to realize however, that impurities do not always increase boiling point, and, in certain rarer cases, can actually cause boiling point to decrease. With this in mind, it is important to know exactly what impurities are being added to a solution in order to determine the final effect the impurities will have on boiling point. Hope this helps!